1. Field of the Invention
This invention relates in general to ultrasonic non-destructive test equipment for locating faults in test specimens.
2. Description of the Prior Art
Modern ultrasonic non-destructive test equipment provides for controlling the sensitivity of the receiver so as to adjust for the far-field characteristics of sound propagation through test specimens. Such units adjust for the ultrasonic beam energy-density characteristics as a function of the distance of sound travel from the transducer and also as a function of the distance for the attenuation characteristics caused by the material composition. Distance amplitude correction units (DAC) can be set for a particular transducer and a particular amount of attenuation. However, for material which have attenuation coefficients that greatly vary automatic distance amplitude correction is highly desireable. My U.S. Pat. No. 3,690,153 which issued on Sept. 12, 1972 is an example of an automatic distance amplitude correction unit which provides automatic correction for the variable response characteristics in ultrasonic tests which are caused by the materials internal structure. Variations in sound intensity result when the material in the path of travel of the ultrasonic beam reflects or absorbs more or less of the ultrasonic energy. The reflected energy in the ultrasonic wavefront that is returned to the transducer in pulse-echo tests is related to the dimensions of the defect encountered by the energy. If the energy arriving at a defect varies then the reflected energy from a defect will vary. Automatic distance amplitude correction apparatus such as described in U.S. Pat. No. 3,690,153 provides a more consistent display of defect images related to defect size by correcting for variable attenuation. The application of automatic distance amplitude correction according to the prior art is limited, however, by the signal strength (I) which is available after the interrogating pulse has travelled back and forth through the material (pulse-echo system). For example, where large attenuation exists due to either or both causes of excessive material cross-section (x) and high attenuation coefficient (.alpha.), as for example, in a 16 inch diameter titanium alloy forging billet, the return echo from the opposite side of the material can be so weak that it has an amplitude less than the background noise and thus it is buried in the noise. Under these conditions, the return echo cannot be used for either manual distance amplitude correction apparatus or for automatic distance amplitude correction apparatus in conventional pulse reflection systems.